symfony
diff --git a/‎src/Symfony/Component/Mime/Header/Headers.php
Copy file name to clipboardExpand all lines: src/Symfony/Component/Mime/Header/Headers.php
+1-1Lines changed: 1 addition & 1 deletion b/‎src/Symfony/Component/Mime/Header/Headers.php
Copy file name to clipboardExpand all lines: src/Symfony/Component/Mime/Header/Headers.php
+1-1Lines changed: 1 addition & 1 deletion
diff --git a/‎src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
Copy file name to clipboardExpand all lines: src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
+9Lines changed: 9 additions & 0 deletions b/‎src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
Copy file name to clipboardExpand all lines: src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
+9Lines changed: 9 additions & 0 deletions
@@ -143,7 +143,7 @@ public function addParameterizedHeader(string $name, string $value, array $param
      */
     public function addHeader(string $name, $argument, array $more = []): self
     {
-        $parts = explode('\\', self::HEADER_CLASS_MAP[$name] ?? UnstructuredHeader::class);
+        $parts = explode('\\', self::HEADER_CLASS_MAP[strtolower($name)] ?? UnstructuredHeader::class);
         $method = 'add'.ucfirst(array_pop($parts));
         if ('addUnstructuredHeader' === $method) {
             $method = 'addTextHeader';
 
@@ -239,6 +239,15 @@ public function testRemoveIsNotCaseSensitive()
         $this->assertFalse($headers->has('Message-ID'));
     }
 
+    public function testAddHeaderIsNotCaseSensitive()
+    {
+        $headers = new Headers();
+        $headers->addHeader('From', ['from@example.com']);
+
+        $this->assertInstanceOf(MailboxListHeader::class, $headers->get('from'));
+        $this->assertEquals([new Address('from@example.com')], $headers->get('from')->getBody());
+    }
+
     public function testToStringJoinsHeadersTogether()
     {
         $headers = new Headers();